same here. old imac 21inch, full of ram, upgraded to Lion today.
if i leave the computer until it goes to sleep, keyboard and magic mouse won't wake it up - snooze light stays on (not blinking/throbbing).
i have to hold button to kill it, then restart. interestingly, it goes back to exactly where it was before, opens safari to the same web page etc.
i'm going to try the solution i saw here that suggests it is still on - so just hit the power button for a second, let it go to sleep for real, then wake it up again with the keyboard/mouse.
i have also unplugged a usb hub.